The present invention relates to methods and apparatus for controlling an amplifier according to an envelope ratio of a signal, and apparatus and methods for determining the peak-to-mean envelope ratio of a signal. In particular, the present invention may be applied to radio frequency (RF) signals such as used in the UMTS system.
RF transmissions from mobile telephones of the present generation typically have a constant envelope shape, and the RF amplifiers operate with constant power. However, in the next generation of mobile telephones, it is intended to operate RF transmissions with a variable envelope. It would be possible to continuously operate the RF amplifier at a high power, sufficient to generate the largest signal output. For example, it would be possible to operate an amplifier at a bias providing a 1 W output capability despite a current output requirement of only 1 mW output power, in order to be capable of providing a 1 W output power when required. However, that would be wasteful of power, as a high standby current (bias) would need to be applied to the RF amplifier at all times. Mobile telephones are typically operated using rechargeable batteries. It is required for the mobile telephone to operate for as long as possible, both in standby and operating modes, from a single battery charge. Therefore, the power consumption of the mobile telephone should be reduced as much as possible.
Some attempts have been made to adapt the bias (standby power consumption) according to the instantaneous value of the output signal envelope. While this would reduce the power consumption of the amplifier, and extend battery life, it has not been found possible to track rapid changes in envelope fast enough to allow efficient RF transmission.